1. Field of the Invention
The present invention relates to an electric motor vehicle having a secondary battery and a generator as an electric power source and, more particularly, to a system of controlling the changeover of the electric power source.
2. Description of the Related Art
Conventionally, an electric motor vehicle in which a motor is mounted as a driving power source is known. FIGS. 8A-8C illustrate examples of arrangement of conventional electric motor vehicles. FIG. 8A shows an example in which two motors are arranged in series to drive front or rear wheels. FIG. 8B shows an example in which the front and rear wheels are respectively driven by two separately mounted motors. FIG. 8C shows an example in which the four wheels are respectively driven by four separate motors. In the drawings, reference numerals 51-62 denote wheels; 63-70, motors; and 71-73, differential gears. A hybrid power source device using a secondary battery (hereinafter referred to as the battery) has been proposed as an electric power source device for driving these motors.
A battery has a large power density, and electric power which can be fetched from it at a time is large. Hence, the battery is advantageous in a case where a high torque is required. However, since the energy density of the battery is small, the battery is disadvantageous in long-distance running. Meanwhile, a generator has a large energy density, so that it is advantageous in long-distance running; however, since its power density is small, the generator is disadvantageous in running which requires a high torque. Thus, since the battery and the generator have mutually complementary characteristics, in a hybrid power source device, the burden of the power density can be borne by the battery, and the burden of the energy density by the generator. As a result, the hybrid power source device constitutes a desirable power source device.
However, with a conventional hybrid power source device, the battery and the generator are fixedly connected in parallel or in series, so that there have been the following drawbacks. Namely, in a case where the battery and the generator are connected in parallel, there has been a problem in that electric current supplied to a load becomes low, thereby making high-speed running difficult. Furthermore, since a decline in the remaining amount of the battery causes the voltage to decline, in the case of parallel connection, there has been the problem that current flows from the generator to the battery, thereby making it difficult to obtain an expected driving force. In addition, in a case where the battery and the generator are connected in series, there has been a problem in that it is impossible to cope with a large current required for starting a vehicle or the like, and it is impossible to recharge the battery by making use of the generator in a case where the load is relatively small as during a standstill, thereby making it impossible to effectively utilize the generator.